The present invention generally relates to an air-fuel ratio control system for adjusting the air-fuel mixing ratio of a combustible air-fuel mixture formed for an automobile internal combustion engine to a predetermined or desired value and, more particularly, to an air-fuel ratio control system effective to carry out the adjustment of the air-fuel mixing ratio on the basis of a feed-back control scheme wherein the concentration of a selected component of exhaust gases emitted from the engine is taken into account in the adjustment of the air-fuel mixing ratio to compensate for change in the air-fuel mixing ratio.
Those skilled in the art are familiar with the fact that, in an automobile exhaust system including a three-way catalytic converter effective to minimize not only CO and HC components but also NOx components of the exhaust gases, the three-way catalytic converter can work at its maximum conversion efficiency if the air-fuel mixing ratio of the combustible mixture to be, or being, supplied to the engine is controlled to a stoichiometric value. A prior art air-fuel ratio control system embodying this concept comprises a composition sensor effective to detect the concentration of a component, for example, oxygen, contained in the engine exhaust gases and to generate an output signal representative of a function of the air-fuel mixing ratio of the combustible mixture burned in the engine, an air-fuel ratio control including a comparator, a proportionating circuit, an integrator, an adder and a duty ratio control circuit and capable of generating an output signal, the magnitude of which is dependent on the output signal from the composition sensor, that is, the concentration of the exhaust gas component, and an actuator provided in an automobile carburetor and operable in response to the output signal from the ratio control circuit to adjust the air-fuel mixing ratio of the combustible mixture being formed in the carburetor.
In this prior art air-fuel ratio control system, where the air-fuel ratio adjustment is carried out by operating the actuator so as to adjust the amount of bleed air to be admixed with fuel before the fuel is mixed with primary air in the carburetor, it is generally considered preferable that the operating range of the actuator, that is, the magnitude of variation in amount of bleed air between the amount of bleed air supplied during the closure of the actuator (the duty ratio being 0%) and the amount of bleed air supplied during the opening of the actuator (the duty ratio being 100%) in the case where the actuator is controlled according to the duty ratio between the open time of the actuator and the closed time of the actuator, should be small in order to attain a precise control of the air-fuel mixing ratio. However, if the above described magnitude of variation is made small, since the combustible air-fuel mixture when the automobile engine is operated at a high altitude and/or under an elevated temperature tends to be enriched due to reduction of the density of atmospheric air with the increased amount of bleed air consequently required, the reference level of the duty ratio (that is, the duty ratio required to maintain the air-fuel mixing ratio at a stoichiometric value) tends to elevate during the condition in which the engine is operated at a high altitude and/or under an elevated temperature, and, if the air-fuel mixture is enriched as a result of a change in engine operating condition, for example, during acceleration, that takes place while the engine is operated at a high altitude and/or under an elevated ambient temperature, any opening of the actuator in an attempt to make the combustible air-fuel mixture leaner does not result in the supply of a sufficient amount of bleed air required to adjust the enriched air-fuel mixture so as to have a mixing ratio equal to the stoichiometric value because the reference level has elevated to such an extent as to reduce the amount of displacement of the reference level towards a level required to make the air-fuel mixture leaner.
On the other hand, the U.S. Pat. No. 4,111,170, patented on Sept. 5, 1978, discloses a technique of adjusting the air-fuel mixing ratio by preventing continuously or in a stepwise fashion air drawn into a carburetor main fuel passage from being undesirably increased with increases in engine load. This patent discloses the use of a controller capable of comparing the output signal from an exhaust composition sensor, which is representative of the sensed air-fuel mixing ratio of the combustible mixture burned in the engine, with a set or desired air-fuel mixing ratio and generating first and second command output signals when the sensed air-fuel mixing ratio is respectively lower and higher than the set or desired air-fuel mixing ratio and a pulse generator operable in response to the first and second command output signals for generating first and second pulse signals, respectively. The first pulse signals are fed to both of first and second electromagnetic flow control valves effective to supply bleed air into the carburetor main and low speed fuel passages, respectively, and are utilized to increase the ratio of the open time of the flow control valves to their closed time to increase the flow of bleed air into the main and low speed fuel passages. On the other hand, the second pulse signals are also fed to the flow control valves, but are utilized to reduce the ratio of the open time of the flow control valves to their closed time in order to reduce the flow of the bleed air into the main and low speed fuel passages.
In any event, the invention of the above mentioned U.S. patent is aimed at solving a problem quite different from that which the present invention is intended to solve, and it is, therefore, believed that the above mentioned U.S. patent is less pertinent to the present invention except for the fact that some component parts of the system of the present invention are disclosed therein.